1. Field of the Invention
The present invention relates to a semiconductor device and a method of fabricating the same, and more particularly, to a semiconductor device having a multi-bit nonvolatile memory cell and to methods of fabricating the same.
2. Description of the Related Art
Flash memories are nonvolatile memory devices that retain memorized information even when the power is interrupted in these devices. EEPROM is a type of flash memory wherein all the memory cells can be erased at once. A significant demand exists for high integration of flash EEPROMs at a lower cost per megabyte (Mbyte). In this regard, a flash EEPROM composed of multi-bit transistors has been developed to meet the above needs in the industry.
A multi-bit transistor sets a multi-stage threshold voltage level, and each threshold voltage level represents a different state. However, for setting the multi-stage threshold voltage level in a flash EEPROM, initial erasing should be performed and the threshold voltage of each memory cell should be less than a minimum threshold voltage. After that, the threshold voltage of each memory cell should be increased to corresponding target threshold voltages in the sequence of a predetermined writing operation. Thus, feedback is required before a writing operation and therefore recording is a time consuming process. In addition, when using this multi-stage threshold voltage method, an operation allowable window becomes narrower. This means that the allowable range of operation is smaller. As such, the yield of fabricated chips is reduced. To widen the operation allowable window, the operating voltage can be increased but this may result in degradation of reliability.